For extractor hoods and other air-purifying devices granular filtering materials are used, particularly adsorptive materials--such as active carbon--or chemisorptive materials; these pourable substances are tipped into filter cassettes etc., which are placed in the path of the airflow so that the air which is to be filtered is made to pass through the porous filter cassette and to flow through the pourable filter material contained in them. Filtering materials are familiar which consist of a mixture of at least one acidic or basic chemisorptive substance and a moisture-storing substance reacting chemically to the entry of external gases and which can be poured (cf. DE-OS No. 24 22 547). This moisture-storing substance can be a water-soluble binding agent which binds the chemisorptive granular substance.
Filter cassettes which work with a pourable filtering material are relatively expensive to produce and have the further disadvantage that various shapes of filter cassette must be made available to fit the various types of extractor hood or other air-filtering devices, thus raising the cost of production and storage considerably.
Filters are also familiar in which sorptive materials are held in powder or granule form in a binding agent and are fixed into the desired form with the aid of the binding agent. These filters can be produced by using a mould or can have the form of filter discs and filter mats. However, the disadvantage here is that the granular sorptive substances are more or less surrounded by the binding agent, resulting in a considerable loss of potential sorptive capability.